The preparation of unsaturated carboxylic esters by means of reaction of alkenes having 2 to 6 carbon atoms with alkanecarboxylic acids having 1 to 6 carbon atoms, in the presence of an oxygenous gas, by means of a continuous homogeneous gas phase process, and in the presence of a heterogeneous noble metal catalyst, is already known. Of particular significance is the preparation of vinyl acetate by a reaction of ethylene with acetic acid and oxygen or oxygenous gases over fixed bed catalysts in the gas phase.
The reactions are effected generally at pressures of 1 to 50 bar, preferably 5 to 15 bar, and at temperatures of 50 to 250° C., preferably 130 to 200° C. Suitable catalysts contain a noble metal component and an activator component. The noble metal component consists generally of palladium and/or compounds thereof. Frequently, gold and/or rhodium or compounds thereof are additionally present. The activator component consists of compounds of elements of main group 1 and/or 2 and/or cadmium. It is also possible for rhenium and/or zirconium compounds to be present. These components are generally applied to support materials, for example silica, aluminium silicates, titanium oxide, zirconium oxide, silicon carbide or aluminium oxide.
The mixture used for the reaction (olefin, alkene, oxygen) generally contains a multiple molar excess of olefin. The ethylene conversion is therefore incomplete in the reaction, and the unconverted olefin must be recycled to the reaction in a circulation system. This recycled olefin-containing gas is referred to as cycle gas. In a saturator connected upstream of the reactor (in vinyl acetate preparation: acetic acid saturator), the olefin-containing cycle gas (in vinyl acetate preparation: ethylene-containing cycle gas) is loaded with the appropriate carboxylic acid (in vinyl acetate preparation: acetic acid) and then loaded with oxygen.
Subsequently, the reaction mixture is passed into the reactor. The hot reaction mixture which leaves the reactor and, in the case of vinyl acetate preparation, consists essentially of unconverted ethylene, unconverted acetic acid, unconverted oxygen, vinyl acetate, water of reaction, carbon dioxide, and inerts introduced with the oxygen and ethylene (for example nitrogen, ethane, methane and argon) is cooled, optionally with upstream connection of a dewatering column. This condenses the majority of the acetic acid, and a portion of the vinyl acetate and of the water. The condensate is separated in subsequent steps, the constituents thereof are isolated and the acetic acid (returned acetic acid) is recycled into the process. The uncondensed residual gas contains principally ethylene, CO2 and inerts and, after CO2 scrubbing and inerts removal, is conducted as cycle gas into the acetic acid saturator.
However, the loading of cycle gas with acetic acid in the acetic acid saturator has the disadvantage that the acetic acid saturator becomes fouled even after short run times. The saturator is generally a column in which dry cycle gas (without acetic acid and water) is first conducted directly into the column from the bottom upward, and acetic acid is metered in. In the lower column region in particular, there is fouling at the introduction site of the dry and hot cycle gas, which impairs the production capacity and even triggers a production shutdown for cleaning.
U.S. Pat. No. 6,420,595 B1 discloses equipping the saturator with a distillation column, withdrawing the fouling with the column bottoms, and removing the impurities from the acetic acid under reduced pressure in an acid recovery unit (ARU), and recycling these impurities into the saturator.
U.S. Pat. No. 7,202,377 B1 ascribes the formation of fouling in the acetic acid saturator to the proportion of returned acetic acid which has been obtained from the condensed proportion of the reaction mixture and which is used in addition to fresh acetic acid to saturate the cycle gas. It is advisable to clean the cycle gas by means of a rectification section attached to the saturation column and to divide the liquid effluxing from the bottom of the saturation column into two substreams, one substream being recycled without workup, and the other substream, after removal of high boilers and polymers, being recycled into a high-temperature thin-film evaporator.